Multivariate ENSO Index Version 2 (MEI.v2)

Overview

The El Niño/Southern Oscillation (ENSO) - a naturally occurring anomalous
state of tropical Pacific coupled ocean-atmosphere conditions - is the
primary predictor for global climate disruptions. These can persist over
several seasons and thereby produce severe regional effects. An appraisal
of the real-time status of ENSO is thus important for a host of climate
services that inform societal responses and trigger policy actions for
water supply, food security, health, and public safety. The MEI, which
combines both oceanic and atmospheric variables, facilitates in a single
index an assessment of ENSO. It especially gives real-time indications
of ENSO intensity, and through historical analysis - provides a context
for meaningful comparative study of evolving conditions.

The bi-monthly Multivariate El Niño/Southern Oscillation (ENSO) index
(MEI.v2) is the time series of the leading combined Empirical Orthogonal
Function (EOF) of five different variables (sea level pressure (SLP),
sea surface temperature (SST), zonal and meridional components of
the surface wind, and outgoing longwave radiation (OLR)) over the
tropical Pacific basin (30°S-30°N and 100°E-70°W). The EOFs are
calculated for 12 overlapping bi-monthly "seasons" (Dec-Jan, Jan-Feb,
Feb-Mar,..., Nov-Dec) in order to take into account ENSO's seasonality,
and reduce effects of higher frequency intraseasonal variability. During
the typical height of ENSO during late Fall/early Winter, the canonical
features of atmosphere and ocean anomalies are shown schematically below
based on a composite of 11 historical El Niño and La Niña events. Key
features of composite positive MEI events (warm, El Niño) include (1)
anomalously warm SSTs across the east-central equatorial Pacific, (2)
anomalously high SLP over Indonesia and the western tropical Pacific and
low SLP over the eastern tropical Pacific, (3) reduction or reversal of
tropical Pacific easterly winds (trade winds), (4) suppressed tropical
convection (positive OLR) over Indonesia and Western Pacific and enhanced
convection (negative OLR) over the central Pacific (Fig. 1a). Key
features of composite negative MEI events (cold, La Niña, Fig. 1b) are
of mostly opposite phase. For any single El Niño or La Niña situation,
the atmospheric articulations may depart from this canonical view.

Fig. 1: Schematic diagram showing the physical mechanisms by which
the SST (shaded), OLR (contours), surface zonal and meridional winds
(vectors), and sea level pressure (represented by "H"
and "L" which indicate the high and low pressure center,
respectively) determine the wintertime Multivariate ENSO Index (MEI)
during (a) El Niño and (b) La Niña events. The schematic
is based on the composite anomalies for November-December (ND) drawn
from 11 warm events and 11 cold events during 1980-2016.

Methods

A new version of the MEI (MEI.v2) has been created that uses 5 variables
(sea level pressure (SLP), sea surface temperature (SST), surface
zonal winds (U), surface meridional winds (V), and Outgoing Longwave
Radiation (OLR)) to produce a time series of ENSO conditions from 1979
to present. The MEI.v2 expands upon the original MEI developed by Wolter and Timlin
(1993) which was calculated using 6 variables as proxies for ENSO relevant
atmosphere and ocean conditions.

In MEI.v2, the fields of SST, SLP, and surface zonal
and meridional winds are obtained from the high-quality JRA-55
global reanalysis (Kobayashi et al. 2015). In contrast, the original
MEI (Wolter and Timlin, 1993) used marine ship observations based on
the International Comprehensive Ocean Atmosphere Data Set (ICOADS)
and used near-surface air temperature as well as SST. The MEI.v2 also
uses observations of OLR from NOAA Climate Data Record (CDR) of Monthly
Outgoing Longwave Radiation (OLR), Version 2.2-1 (available from NOAA National Centers for Environmental Information
(NCEI)); whereas the original MEI used ICOADS cloud cover fraction
data. To produce the MEI.v2, all variables are interpolated to a common
2.5° latitude-longitude grid and standardized anomalies are computed
with respect to the reference period of 1980-2018. As with the original
version of the MEI (Wolter and Timlin, 2011), the MEI.v2 is calculated
as the leading principal component (PC) time series of the Empirical
Orthogonal Function (EOF) of the standardized anomalies of the above 5
combined variables over the tropical Pacific during 1980-2018. The EOF
analysis is based on the covariance matrix and the analysis domain is the
same as for the original MEI (30°S-30°N and 100°E-70°W,
excluding the Atlantic Ocean and the land regions). A latitudinal
weighting prior to the EOF analysis is applied.

The EOF analysis for MEI.v2 is conducted for 12 partially overlapping
2-month "seasons" (e.g., Wolter and Timlin, 1993). To obtain
MEI.v2 values before 1980 and after 2018, standardized anomalies maps
relative to the 1980-2018 reference period are projected onto the leading
EOF pattern. Because the OLR record starts in January 1979, the DJ 1979
MEI.v2 value is based on January 1979 OLR data only.

MEI.v2 Values

This page will be updated automatically by the 10th of each month. For the period between 1979 and 2018, the ICOADS-based MEI and MEI.v2 are correlated at 0.95. Last data update: 9 May 2019

Current Value:

Historical Values

MEI.v2 Evolution of Historical ENSO Events

Temporal evolution of El Niño and La Niña events indicated by the MEI.v2. Shown are the top 5 warm and cold non-overlapping events since 1979. Year 0 corresponds to the year of the event in the legends. Click on image to enlarge.

Correlation Maps

Maps of correlation coefficients between MEI.v2 and anomaly time series
of the variables from which the MEI is derived for bi-montly overlapping seasons. Click on an image to enlarge.